Previous work, outlined below, suggested the presence in cardiac and skeletal muscle of an intrinsic mechanical control system. Some implications of the presence of such a system will be examined by determining heat production and by further mechanical studies. Heat production during twitches of cardiac muscle will be analysed using rings of the frog ventricle, and the effect of length changes will be determined. Because of widely varying views, the viscous properties of resting smooth muscle will be reinvestigated.